Make sure you went off the right mark on the harmonic balancer. I ran into that earlier this year, my harmonic balancer has 2 indentions on it and I was using the wrong one getting screwy timing readings. Once I realized what happened it all made sense.

^^^^Yup. And I've run into some engines with two timing tabs mounted to engine...make sure you use the right one.

BTW: back in the day...with inefficiently designed large domes on pistons and the OE inefficient combustion chamber, it wouldn't be uncommon to run from 42 all the way up to 48* BTDC for total timing on BBC's at the race track. Sounds crazy, but it was almost the norm back then. Obviously newer marine is different, but wanted to state that fact before anyone thinks that timing figure is unheard of.

Oh, and sometimes people without advance timing lights will make a paint stripe on the balancer at 8-10*BTDC so they can line that up with the big indention (0*) to se it more clearly.

This can mix up the next person of course. So, make sure if it has a painted line on the balancer, it is actually on the 'cut or grooved' line on the balancer.

I've got a drawer full of old spark plugs that I broke the insulator off of, tapped the ID and put a bolt into to serve as a piston stop for TDC reference. When I have a new motor in front of me to work on, and it involves anything related to timing (ignition, cams, etc) the first thing I do is verify the TDC mark on the damper and degree tag. Some have been off a surprising amount. Knowing that you have accurate TDC indicator is good.

Knowledge is power.

Thx for all the good replies!

Good to know I can use #6 cyl, however I would still have to pull all the plugs to turn the eng over manually, especially with a high compression eng.
Wonder if I could just use a bolt with the same threads as the spark plug? Are plug threads special??

I'm using an analog adv timing light. Had it for years and seems to be working fine.

The damper has yellow paint at TDC (in the groove).

Only 1 timing tab on these engs.
The heads are the large rectangular port Merlins. Not state of the art by any means but not ancient either.

I like the idea of comparing the key way to the TDC mark. I can compare with the port eng. Will need to take a look and see if the key way is even visible with the limited space in front of the engs. Even if I can see the key way, I'm not positive I can eyeball ~ 6 degs of slippage at such an odd angle.

It is a 14 mm spark plug thread. O2 sensors use it too.

Went out and eyeballed the situation again looking to see if I had a chance of being able to see the key way. Can't tell for sure until I remove the crank pulley but it doesn't look good. However, I did notice the crank pulley has a raised line cast into the metal that lines up perfectly with the TDC mark on the damper.
Checked the other eng and sure enough it has the same raised line and it's also lined up perfectly. Anyone know about this cast line on the crank pulley? I believe it's the original pulley from when it was a gen-4 365hp Mag (1990).

Anyway, this makes me feel a little better. Maybe these engs do like a lot of advance. The prev owner specifically told me not to change the timing but didn't say why. Don't know if the MSD distributors were rusted or not when he had it. I'll double check the timing and maybe bump it up from 34 to 36 degs and see how it goes.
I took the boat out by myself and made a WOT run. Both engs are at 4800 RPM and it still gets into chine walk territory so it will be plenty fast enough for the family over the 4th.